The mishna in Shabbos 4(1) tells us which materials may not be used to insulate foods for Shabbos because they "add heat". One of these materials is salt. Can anyone explain how salt can add heat other than by saying that the salt in the time of the Mishna was not the same as our salt?

in Baba Basra 17:1 (mechon-mamre.org/b/l/l4302.htm) it is also mentioned that salt makes same type of destruction to wall as olive waste, so there we see also that it adds heat.
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jutkyAug 18 '11 at 8:49

I don't understand why the question is limited to only salt. The Mishna bans peat, foliage, salt, lime or sand, straw grape-skis, soft flocking or moist-herbage. The question should be how any of these materials add heat.
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zaqAug 18 '11 at 13:24

Organic material can get quite hot from fermentation, as Shalom noted. But salt would not have any such effect. (Presumably, though, the question would apply to lime and sand.)
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DaveAug 18 '11 at 18:17

4 Answers
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I vaguely recall seeing some discussion about the whole "heat adding" thing years ago (maybe an old AOJS article?), but unfortunately don't have the details. Honestly, I'm still not certain how to understand the mishna. But off the top of my head ...

Shalom, thank you for the suggestions. I look forward to comments on them. I found that the heat required to dissolve salt is around +4 kcal/mole (not enough I think.)
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Avrohom YitzchokAug 17 '11 at 21:09

New answer: Insulation materials that "add heat" are heated up and then either put around a pot, or a pot is put on top of them, resulting in the pot getting hotter than it was. This is in contrast to a towel that only keeps the heat of the pot in. (Shabbos, 34a)

They are prohibited to use even if the pot is set in them before Shabbat, because you can put coals into these materials and then on Shabbat you may stoke them. (Shabbos, 34b)

Before Shabbat, a pot can be removed from the oven and put in something to keep it warm, but not if the insulation is one of these materials which 'adds heat': olive-peat, foliage, salt, lime or sand (wet or dry), straw, grape-skins, soft-flocking or moist-herbage.

These materials either allow coal to be added, [or they're a certain level of 'hot' to begin with, (hot as olive-peat is after making oil) and then used to heat an approved insulation].

Two situations Rabbi Dovid Grossman gives in his Daf yomi shiur where these materials "add heat":

If you cover a pot before shabbat with something, like sand or salt, you can put a coal inside the pile of salt before shabbat, and then you may come to rake the coals to make them hotter after shabbat starts. (The salt in this situation is a material which makes it possible to "add heat" in the form of a hot-coals.)

He says this because of Shabbos 34b : one must not insulate in a substance which adds heat, for fear lest he put it away in hot ashes (or sand, or salt...) containing a burning coal. Ashes "add heat", because you can put a coal in it.

So any type of material which "adds heat" was banned because you may put hot coals in it and then stoke it on Shabbat.

2. A pot wrapped in a towel (an allowed form of insulation) cannot be put on hot-olive-peat (or salt...) because the peat "adds heat" to the towel, which is already insulating the pot.
(Hot olive peat is used as the example because there is a question of it is hot enough.) < Striken because the first reason applies more to salt.

Could you please clarify? I didn't understand any of your points.
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DaveAug 17 '11 at 21:52

Salt is a form of insulation that adds heat for three reasons. First, it makes it possible to add a coal (a pot is buried in salt before shabbat, then Shabbat starts and you decide to bury a coal in the salt to make the pot hotter - it's assur to add the coal). Second it can add heat to an approved form of insulation (a pot wrapped in a towel and buried in hot-salt - it is assur to heat the towel). Third, the salt traps too much heat, which is called 'added' because it would have otherwise escaped.
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zaqAug 17 '11 at 22:03

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Do you have any source whatsoever for this novel interpretation?
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DaveAug 18 '11 at 3:08

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Novel? It's the reason the gemara gives for not using these materials which add heat, including salt.
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zaqAug 18 '11 at 11:40

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(cont.) The Gemara does not mean that one might put a coal in the salt. Besides not fitting into the wording of the Gemara, that explanation would not be specific to "mosif hevel"; if people are liable to add coals to insulation, we should be concerned even when the insulating material is not adding heat.
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DaveAug 18 '11 at 22:28

Can't say for sure, but salt lowers the temperature at which water freezes (which is why in cold areas, people clear the snow and ice from their walks using salt). This can be casually observed as a type of heat even if it does not have a burning sensation.

This is what I was going to answer. But I'll expand on it, unless you want to.
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aviAug 18 '11 at 8:47

Be my guest. Anything else I had to say was just distracting from the above very simple answer.
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YDKAug 18 '11 at 13:11

Eretz Yisrael was not exactly a cold area, and even in Bavel, I doubt they used salt to clear their driveways. In any event, you're suggesting that salt was mistakenly perceived as adding heat; if so, you could have explained much more simply that salt was viewed as "hot" because of its strong flavor. (Ironically, by the way, adding salt to ice actually lowers its temperature.)
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DaveAug 18 '11 at 18:10

@Dave, although I don't know if my answer has validity, I'm not sure if we're on the same page. Regarding temperature Mid-Eastern temperatures, all you need is someone to drop a rock of salt on snow, which isn't rare even for the location. Re: strong flavor- I don't see how one would relate that to heat for bishul. Re: ice temperature- it is certainly lowering the surrounding temperature as the ice draws in excessive heat which causes it to melt.
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YDKAug 18 '11 at 19:42

All I'm saying is that "salt melting ice" was not a common occurrence then, certainly nothing that would be obvious to the masses. The strong flavor logic is related to מליח כרותח with regard to issur v'heter. It's not a stretch to say that this "resichus" was interpreted as an energy that could heat a pot. Re: ice lowing temperature, it's a reaction similar to an a/c evaporator, the state change causes it to absorb energy (or something like that). See e.g. here: antoine.frostburg.edu/chem/senese/101/solutions/faq/…
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DaveAug 18 '11 at 21:37

As YDK said, salt lowers the freezing point of Water. It also raises the boiling point. This means that you need 'less heat' to boil water with salt than you do without salt. In the times of the Mishna, this can be understood to be that the salt itself 'adds it's own heat'

That is, if you look at this from a simple observation point of view, instead of a modern science language...

If I have a pot of water it takes 10 burning coals to boil the water.
If I add salt to the pot, then I only need 9 burning coals to boil the water. Therefore, halachicly, the salt, "adds heat"

Another way to say this.. if my fire is no hot enough, I can add salt to make the food cook better / faster.

This means that in general, salt 'insulates' a mixture that is wet. From the perspective of the halacha, salt reduces the amount of heat needed to cook things, and people might therefore use the salt to help a wet mixture heated by coals or other fires to cook better. This will then lead people to adjust the coals themselves, as all forms of insulation are likely to do.

Your science seems fine, but the mishna is talking about adding heat in the context of insulating hot food before and on Shabbos.
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Avrohom YitzchokAug 18 '11 at 16:34

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I apologise, I gave a poor explanation. What I meant was a phrase in the Bartenuro on "Rabbi Yehuda oseir bedako". Bartenuro says (my translation) "the reason that Chazal forbade insulating before shabbos with something that adds heat is because it may lead him to stir up the coals under the pot eventually on Shabbos". The connection to the coals led me to think that the mishna is speaking about really adding heat and not requiring less heat to boil a salt solution.
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Avrohom YitzchokAug 19 '11 at 9:11

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@Avrohom I see, that's an important piece of information to add to your question. (And not directly part of the mishna) And I can see now why my answer was not satisfactory, I will update it.
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aviAug 19 '11 at 10:10

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@avi, the Bartenura does not say "under the pot". He is using the phrase shema yechate bagechalim in reference to the remetz which was commonly used as a heat increasing insulation. Because of remetz, the chashash of shema yechate was applied across the category of heat increasing insulation.
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YDKAug 21 '11 at 3:45

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@avi - Now that you've modified the beginning of the answer, the rest of it is kind of contradictory.
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DaveAug 21 '11 at 4:07